1. Field of the Invention
The present invention relates to an image forming apparatus including a structure in which two members are coupled to each other by a fixing member.
2. Description of the Related Art
In a structure of a copying machine, a FAX, a printer, or the like, multiple members are combined, and those members are fixed to each other by fastening means such as welding, screw fastening, or a rivet.
For example, in a case of welding, members are melted for fixation, and hence the coupled portion shows high strength. However, the welding requires large-scale equipment, and hence cost increases. Further, there is a problem of deterioration of accuracy due to heat deformation. Instead of the welding, screw fastening or rivet fastening may be used. Usually, in a case of fastening two members to each other with a screw or rivet, a diameter of a hole through which the screw or rivet is to be inserted is made larger than an outer diameter of an insertion portion of the screw or rivet. With this, a sufficient allowance is ensured between the insertion portion and the hole at the time of insertion. The reason is to insert the rivet or screw easily, and to prevent a cost increase resulting from an increase in dimensional accuracy. However, in a case of the screw fastening or rivet fastening, when the diameter of the hole has an allowance, a gap corresponding to the allowance remains after the fastening. Thus, when large load is applied to the coupled portion, there has been a problem in that slippage occurs at the coupled portion.
In order to solve this problem, the following blind rivet has been proposed. A step is formed on each of a mandrel of the blind rivet and an inside of a body for receiving the rivet, and the outer diameter of the stepped portion of the mandrel is increased toward the stepped portion of the body in a fixing process. With this configuration, it is possible to eliminate a gap that occurs between the members after fixation, and to effect fixation in which both the members coupled to each other with the blind rivet do not change in relative positions with time. In order to increase versatility of this technology, Japanese Patent Application Laid-Open No. 2004-286072 proposes the following configuration. The tubular projection is provided on one fixing member, the hole portion is provided in the other fixing member, and the screw having a diameter larger than the inner diameter of the tubular projection is press-fitted into the inside (hollow portion) of the tubular projection fitted into the hole portion. With this configuration, by press-fitting the screw, the tubular projection can be expanded, and the gap between the tubular projection and the hole portion can be eliminated. In addition, the related art includes technologies disclosed in Japanese Patent Application Laid-Open No. H10-299735, Japanese Patent Application Laid-Open No. H10-306813, Japanese Patent Application Laid-Open No. H10-306814, and Japanese Patent No. 4244316.
In the above-mentioned conventional examples, two components are fixed to each other to prevent backlash from occurring at the fixing portion of the two components, and hence a fixing accuracy varies due to a projection shape before fixation, a position of a hole shape, and deformation generated in an expanding process.
Assuming that the two components are processed by general press working, the following variation occurs. In a case of forming the tubular projection, primary processing of performing a punching step and secondary processing of drawing the projecting portion are carried out. In the primary processing, portions required for a positional accuracy are processed simultaneously, with the result that the portions are finished to have highly accurate dimensions (for example, ±0.1 or less). In the secondary processing, the projecting portion is processed to have a drawn shape, and hence the dimensional accuracy tends to deteriorate (for example, ±0.2 or less). With the result of the primary processing and the secondary processing, the required dimensional accuracy becomes a position variation approximate to the total value (about 0.3 if the above-mentioned two accuracies are obtained).
In addition, as a fixing principle, there is required deformation due to the expanding process for reducing backlash. This deformation generates variations in fixing positions of the two components, and its numerical regulation is not easy. There is no problem if the two components are not required for a high fixing accuracy. However, as a fixing accuracy of a structure of an image forming apparatus, the accuracy of a single component with the result of the primary processing needs to be reproduced at it is. In particular, in a color LBP using a belt mechanism, in order to maintain stability of running of the belt and to set color shift among four colors within a certain allowed value or less, a positional accuracy of the structure is used as an important parameter.